Device for displaying a waveform with variable persistence and method of providing the same

ABSTRACT

A device includes a memory storing a persistence bit for each of a plurality of pixels of a display device, the persistence bit having a first value when a corresponding pixel should be illuminated for displaying a persistent image, and having a second value when the corresponding pixel should not be illuminated for the persistent image; a pseudorandom pixel value generator which during each video frame receives a seed value and generates pseudorandom pixel values for the plurality of pixels, each pseudorandom pixel value being not greater than a specified variable persistence value; a frame value generator outputting a frame value for each video frame; and a match detector which, during each video frame, compares the frame value to the pseudorandom pixel values for the plurality of pixels, and for each pixel where the comparison indicates a match, makes the persistence bit for the corresponding pixel have the second value.

BACKGROUND

Many measurement instruments include a display device, and/or provideoutputs for a display device, for displaying waveform imagescorresponding to the instrument's measurement of an input signal appliedto the instrument. Examples of such instruments include oscilloscopes,logic analyzers and digital spectrum analyzers. In these instruments,waveform images are drawn onto a rasterized display device at thedisplay device's video frame rate—for example, 60 frames per second.

The “persistence” of a waveform image describes the length of time thatthe waveform image continues to be displayed in whole or in part on thedisplay device before it is erased. If the waveform image is erased oneframe after it is drawn, then the waveform image has a minimumpersistence (also sometimes referred to as non-persistence). At theother extreme, if the waveform image was never erased from the displaydevice, then the waveform image would have infinite persistence. Betweenthese two extremes lies a range of finite persistence for a displayedwaveform image.

FIG. 1 illustrates an example oscilloscope 100 with a variablepersistence. Oscilloscope 100 includes inputs 110 for receiving signals,a display device 120 for displaying one or more waveform imagescorresponding to the instrument's measurement of one or more inputsignals, and a control section 130 having a plurality of control inputs,including variable persistence control inputs 135 (e.g., rotatableknobs), one for each channel of the oscilloscope.

FIG. 2 shows a functional block diagram of portions of one example of adigital oscilloscope 200. Oscilloscope 200 includes probe 210, avertical system 220, a trigger system 230, a horizontal system 240, anacquisition system 250, a digital display system 260, and a displaydevice 270.

Often it is desirable to give a user of a measurement instrument one ormore control inputs (e.g., variable persistence control inputs 135 inoscilloscope 100) for receiving a user input for controlling oradjusting the persistence of displayed waveform images. When the userselects a small persistence value (e.g., 0.5 seconds), then the waveformimage remains displayed for only a short amount of time, providing alively, responsive display. Longer persistence times allow the waveformimages to linger on the display, allowing a user more time to analyzethe waveform.

In existing instruments, this flexibility comes at a price in terms ofmemory requirements imposed on the instrument. The common approach todisplaying waveforms for a variable amount of time on a display devicein a digital oscilloscope is to keep track of how long each pixel of thewaveform image has been displayed and to decrement this time to zero ata regular rate. One common implementation stores in memory a“time-left-on-screen” value for each pixel of the display device. Foreach video frame, this value is read from memory for each pixel. If thevalue is non-zero, then the pixel is displayed, the value isdecremented, and the new value is written back into memory. This isrepeated for every channel of the oscilloscope for which a persistentwaveform image is displayed. This requires a high-speed multiple-bitdecrementer for each channel. This also requires a read-modify-writememory operation for each pixel in each video frame. With a displaydevice having a large number of pixels, this translates to a largeamount of display memory with a high display memory bandwidth, resultingin relatively expensive display memory system.

It would be desirable to provide another solution for displaying awaveform with variable persistence in display device associated with ameasurement instrument that could be less complex and less expensive.

SUMMARY

In an example embodiment, a device comprises: a memory configured tostore a persistence bit for each of a plurality of pixels of a displaydevice, the persistence bit having a first value when a correspondingpixel should be illuminated for displaying a persistent image, andhaving a second value when the corresponding pixel should not beilluminated for the persistent image; a pseudorandom pixel valuegenerator configured to receive a seed value for each video frame of thedisplay device and in response thereto to generate pseudorandom pixelvalues for the plurality of pixels during each video frame, eachpseudorandom pixel value being not greater than a specified variablepersistence value corresponding to a desired persistence of thepersistent image; a frame value generator configured to output a framevalue for each video frame of the display device, each frame value beingnot greater than the specified variable persistence value; and a matchdetector configured to compare the frame value to the pseudorandom pixelvalues for the plurality of pixels during each video frame, and for eachpixel where the comparison indicates a match, to make the persistencebit for the corresponding pixel in the persistence display memory havethe second value.

In another example embodiment, a method comprises: storing in a memory apersistence bit for each of a plurality of pixels of a display device,the persistence bit having a first value when a corresponding pixelshould be illuminated for displaying a persistent image, and having asecond value when the corresponding pixel should not be illuminated forthe persistent image; establishing a variable persistence valuerepresenting a number of video frames of persistence for displaying thepersistent image; and during each video frame of the display device:generating a frame value not greater than the variable persistencevalue, generating for each of the plurality of pixels a pseudorandompixel value not greater than the variable persistence value, and foreach pixel of the display device whose pseudorandom pixel value matchesthe frame value, making the persistence bit for the corresponding pixelin the persistence display memory have the second value.

BRIEF DESCRIPTION OF THE DRAWINGS

The example embodiments are best understood from the following detaileddescription when read with the accompanying drawing figures. In fact,the dimensions may be arbitrarily increased or decreased for clarity ofdiscussion. Wherever applicable and practical, like reference numeralsrefer to like elements.

FIG. 1 illustrates an example oscilloscope with a variable persistence.

FIG. 2 shows a functional block diagram of one example of a digitaloscilloscope.

FIG. 3 shows a flowchart of one embodiment of a process for processingdata to display a variably persistent image on a display device.

FIG. 4 illustrates one embodiment of a method of displaying a variablypersistent image on a display device.

FIG. 5 shows a high level functional block diagram of one embodiment ofa device for processing data to display a variably persistent image on adisplay device.

FIG. 6 shows a detailed block diagram of one embodiment of a device forprocessing data to display a variably persistent image on a displaydevice.

DETAILED DESCRIPTION

In the following detailed description, for purposes of explanation andnot limitation, example embodiments disclosing specific details are setforth in order to provide a thorough understanding of an embodimentaccording to the present teachings. However, it will be apparent to onehaving ordinary skill in the art having had the benefit of the presentdisclosure that other embodiments according to the present teachingsthat depart from the specific details disclosed herein remain within thescope of the appended claims. Moreover, descriptions of well-knownapparati and methods may be omitted so as to not obscure the descriptionof the example embodiments. Such methods and apparati are clearly withinthe scope of the present teachings.

Unless otherwise noted, when a first device is said to be connected to asecond device, this encompasses cases where one or more intermediatedevices may be employed to connect the two devices to each other.However, when a first device is said to be directly connected to asecond device, this encompasses only cases where the two devices areconnected to each other without any intermediate or intervening devices.Similarly, when a signal is said to be coupled to a device, thisencompasses cases where one or more intermediate devices may be employedto couple the signal to the device. However, when a signal is said to bedirectly coupled to a device, this encompasses only cases where thesignal is directly coupled to the device without any intermediate orintervening devices.

FIG. 3 shows a flowchart of one embodiment of a process 300 forprocessing data to display a variably persistent image on a displaydevice—for example a variably persistent waveform image for ameasurement instrument such as a digital oscilloscope, a logic analyzer,a digital spectrum analyzer, or other measurement instrument whichdisplays data or measurement results on a display device.

In step 310, a variable persistence value V is selected, for example inresponse to a user input received via a control input of a measurementinstrument, such as a digital oscilloscope. For example, a user of ameasurement instrument may operate a control input of the measurementinstrument to provide a user input to select a time period (e.g., fiveseconds) of desired persistence for the displayed data traces. Inresponse to the user input, the measurement instrument may determine thevariable persistence value V in units of video frames of the measurementinstrument's display device. For example, if the user input selectsthree seconds of persistence, and if the display device's video framerate is 60 frames/second, then V would be 180. In practice, the variablepersistence value V can range from zero to a maximum persistence valuesupported by the measurement instrument. In some embodiments, themeasurement instrument may also have a separate control input forallowing a user to select “infinite persistence” wherein the waveformimage remains displayed permanently as long as the control input isenabled. Further details of setting the variable persistence value Vwill be described in greater detail below, particularly with respect toFIG. 4.

In a step 320, a frame value generator sets a frame value F to zero. Aswill be explained in greater detail below, the frame value F isincremented once for each video frame until it reaches a maximum framevalue of V−1.

In a step 330, a pseudorandom pixel value P_(XY) is assigned to eachpixel (X, Y) of the display device, where X designates a row and Ydesignates a column of a display device. In various embodiments, thepseudorandom pixel value P_(XY) may have any integer value between 0 andV−1. For example, in some embodiments a pseudorandom pixel valuegenerator may include a pseudorandom number generator (PRNG) which isreset once per video frame, and which generates a new pseudorandom pixelvalue P_(XY) at the pixel clock rate for each pixel (X, Y) of thedisplay device. In some embodiments, for each displayed pixel (X, Y),the pseudorandom pixel value generator generates the exact samepseudorandom pixel value P_(XY) for each video frame.

In a step 340, each pixel (X, Y) of the display device whosepseudorandom pixel value P_(XY) matches or is equal to the frame value Fis turned off for the persistent image that is being displayed.

In a step 350, it is determined whether a new video frame is beginning.If not, the process waits at step 350 until a new frame starts. If so,then the process proceeds to step 360 where it is determined if theframe value=V−1.

If in step 360 it is determined that the frame value does not equal V−1,then in step 370 the frame value F is incremented and the processreturns to step 340 where the pixels (X, Y) whose pseudorandom pixelvalue P matches or is equal to the new frame value F are turned off forthe persistent image that is being displayed. If in step 360 it isdetermined that the frame value does not equal V−1, then the processreturns to step 310 where the frame value generator is reloaded andreset to zero.

To better visualize how this works, FIG. 4 illustrates one embodiment ofa method 400 of displaying a variably persistent image on a displaydevice. FIG. 4 shows an example of a 5×6 array 410 of pseudorandom pixelvalues P_(XY) for a display device 420 having a corresponding 5×6 arrayof pixels (X, Y). In the illustrated example, it is assumed that: thespecified variable persistence value V is set equal to 5; a pseudorandompixel value generator generates the pseudorandom pixel values P_(XY)shown in array 410 on the top line of FIG. 4; and the persistent imageto be displayed is as shown on display device 420 on the second-from-topline of FIG. 4.

As illustrated in FIG. 4, during a video frame “0”, the frame value F is0, and each pixel (X, Y) in display device 420 whose correspondingpseudorandom pixel value P_(XY) in array 410 is 0 is turned off fordisplaying the variably persistent image in display device 420. Duringvideo frame “1”, the frame value F is 1, and each pixel (X, Y) indisplay device 420 whose corresponding pseudorandom pixel value P_(XY)in array 410 is 1 is turned off for displaying the variably persistentimage in display device 420. The process continues up to video frame “4”where the frame value F is equal to V−1, at which time all of the pixels(X, Y) in display device 420 have been turned off for the variablypersistent image.

It is noted that the displayed variably persistent image is erasedgradually over the V=5 display frames. Also, due to the pseudorandomdistribution of pseudorandom pixel values P_(XY) assigned to the pixels(X, Y) in display device 420, this erasing is “spread out” across thedisplay device 420. In the example shown in FIG. 4, ⅕^(th) of the pixelsare erased during each video frame, and every pixel is guaranteed to beerased with V=5 frames. Such behavior may be considered to be desirablein comparison to waiting until the fifth frame and then erasing thepersistent image all at once, which in contrast to the methodillustrated in FIG. 4 provides no indication of the “age” of thepersistent image. As shown in FIG. 4, as the number of pixels thatremain turned on for the persistent image diminishes from video frame tovideo frame, the overall brightness of the persistent image decreasesgradually so that the persistent image appears to a viewer to fade awayinto the background.

It should be noted that during the process 300, in general newadditional live data traces could be gathered by the measurementinstrument, and therefore would be added into the persistence memory.These data traces can be thought of as entering the loop in process 300at an arbitrary frame number. That is, the loop the process 300 iscontinuously running or being executed, and this new traces will becleared away with the V (e.g., 5) display frames. Advantageously, bythis arrangement the system does not have to keep track of the “age” ofa persistent image and therefore does not require an expensive memory tohold or store this age. As a result, a one-bit-per-pixel persistencedisplay memory can be employed which holds data that would always be amixture of traces of varying ages at any given time.

FIG. 5 shows a high level functional block diagram of one embodiment ofa device 500 for processing data to display a variably persistent imageon a display device—for example a variably persistent waveform imagegenerated by a measurement instrument. Device 500 includes a frame valuegenerator 510, a pseudorandom pixel value generator 520, a matchdetector 530, a memory 540 (hereinafter referred to as persistencedisplay memory 540), and a timing generator 550.

In operation, persistence display memory 540 stores a persistence bitfor each pixel of a display device, the persistence bit having a firstvalue (e.g., “1”) when a corresponding pixel in the display deviceshould be illuminated for a persistent image representing a measurementby the measurement instrument, and having a second value (e.g., “0”)when the corresponding pixel should not be illuminated for thepersistent image.

Frame value generator 510 generates a frame value F for each videoframe. The frame value F may assume any value from 0 to V−1, where V isa variable persistence value representing a number of video frames ofpersistence for displaying the persistent image on the display devicecorresponding to a desired persistence of the persistent image, whichmay be a persistence selected by a user via a control input of themeasurement instrument. In one embodiment frame value generator 510 maycomprise an N-bit counter which is loaded with variable persistencevalue V and is incremented once per video frame. In that case, N isselected to support the maximum allowable variable persistence for thedevice 500. For example, using an N=10 bit counter, and a video framerate of 60 frames/second, frame value generator 510 can support2¹⁰/60=17 seconds of maximum variable persistence. For example, if auser input indicates a desired persistence of two seconds for thepersistent image, then V would be set equal to 2*60=120 and in responseto a timing signal (e.g., a vertical sync signal) from timing generator550, the N-bit counter of frame value generator 510 would count up from0 to 119 (or alternatively, count down from 119 to 0).

Pseudorandom pixel value generator 520 receives a seed value for eachvideo frame of the display device and in response thereto generates apseudorandom pixel value P_(XY) for each pixel (X, Y) of the displaydevice during each video frame. In one embodiment, in response to atiming signal (e.g., a vertical sync signal) from timing generator 550,pseudorandom pixel value generator 520 is reloaded with the same seedvalue for each video frame of the display device. In that case, for eachpixel (X, Y) of the display device pseudorandom pixel value generator520 generates the same pseudorandom pixel value P_(XY) for every videoframe. Each pseudorandom pixel value P_(XY) is less than the specifiedvariable persistence value V.

During each video frame, match detector 530 compares the frame value Fto the pseudorandom pixel value P_(XY) for each pixel (X, Y), and foreach pixel (X, Y) where the comparison indicates a match, match detector530 generates an output which makes the persistence bit for thecorresponding pixel (X, Y) in persistence display memory 540 have thevalue (e.g., the second value or “0”) that indicates that thecorresponding pixel in the display device should not be illuminated forthe persistent image. As timing generator 550 causes frame valuegenerator 510 to generate a new frame value F for each video frame, thematch detection is repeated each video frame and produces a differentresult in each video frame for each pixel (X, Y) of the display device,gradually turning off all of the pixels for the persistent image andthereby erasing the persistent image.

FIG. 6 shows a detailed block diagram of a device 600 for processingdata to display a variably persistent image on a display device. Device600 includes: a memory 610 (hereinafter referred to as live data displaymemory 610), a memory 620 (hereinafter referred to as one-bit-per-pixelpersistence display memory 620), one or more row/column counters 630, apseudorandom pixel value generator 640, a counter 650 (hereinafterreferred to as frame counter 650), a match detector 660, an imagedetector 670, and logic circuits 680. Pseudorandom pixel value generator640 includes a pseudorandom number generator (PRNG) 642 and a modulusunit 644. In one embodiment, PRNG 642 may comprise a P-bit shiftregister with an exclusive NOR gate providing feedback from the registeroutputs to its input.

Live data display memory 610 and one-bit-per-pixel persistence displaymemory 620 each include one memory location corresponding to one of thepixels of a display device on which the variably persistent image is tobe displayed. In general, live data display memory 610 stores many bitsfor each pixel, for example representing an intensity and/or color withwhich a corresponding pixel is to be illuminated. In a beneficialfeature, one-bit-per-pixel persistence display memory 620 stores exactlyone persistence bit for each pixel of the display device. Thepersistence bit has a first value (e.g., “1”) when a corresponding pixelin the display device should be illuminated for a persistent imagerepresenting a measurement by the measurement instrument, and has asecond value (e.g., “0”) when the corresponding pixel should not beilluminated for the persistent image.

In operation, row/column counters 630 receives a pixel clock and inresponse thereto during each video frame sequentially outputs row andcolumn addresses corresponding to a full scan of the display device. Therow and column addresses are provided to live data display memory 610and one-bit-per-pixel persistence display memory 620 to therebysequentially scan the memory locations corresponding to the pixels ofthe display device. Row/column counters 630 also outputs a video framesynchronization signal (e.g., a vertical sync VSYNC) that includes anindication (e.g., a pulse or strobe) of each new video frame.

As each memory location of live data display memory 610 corresponding toeach pixel of the display device is accessed in response to row/columncounters 630, image detector 670 detects whether or not the pixel isilluminated and if so, sets a corresponding one-bit-per-pixelpersistence display memory 620 to have the first value (e.g., “1”)thereby indicating that the corresponding pixel in the display deviceshould be illuminated for a persistent image representing a measurementby the measurement instrument.

Frame counter 650 is loaded with the persistence value V representing anumber of video frames of persistence for displaying the persistentimage on the display device corresponding to a desired persistence ofthe persistent image, which may be a persistence selected by a user viaa control input of the measurement instrument. In response to the videoframe synchronization signal from row/column counters 630, the counteris incremented (or in an alternative embodiment is decremented) togenerate a frame value F from 0 to V−1 for each video frame.

Pseudorandom pixel value generator 640 receives a constant seed valuethat is reloaded for each video frame of the display device in responseto the video frame synchronization signal from row/column counters 630.In response thereto, pseudorandom pixel value generator 640 generates apseudorandom pixel value P_(XY) for each pixel (X, Y) of the displaydevice during each video frame in response to a timing signal (e.g., avertical sync signal) from row/column counters 630. In particular, inresponse to the pixel clock PRNG 642 generates a new pseudorandom numberonce for each pixel period. Modulus unit 644 receives the specifiedvariable persistence value V and in response thereto converts thepseudorandom number from PRNG 642 into a corresponding pseudorandompixel value P_(XY). Each pseudorandom pixel value P_(XY) is an integerthat is less than the specified variable persistence value V. Becausepseudorandom pixel value generator 640 is reloaded with the same seedvalue for each video frame of the display device, for each pixel (X, Y)of the display device pseudorandom pixel value generator 640 generatesthe same pseudorandom pixel value P_(XY) for every video frame.

During each video frame, match detector 660 compares the frame value Fto the pseudorandom pixel value P_(XY) for each pixel (X, Y), and foreach pixel (X, Y) where the comparison indicates a match, match detector660 supplies an output signal to logic circuits 680 to cause thepersistence bit for the corresponding pixel (X, Y) in one-bit-per-pixelpersistence display memory 620 have the value (e.g., the second value or“0”) that indicates that the corresponding pixel in the display deviceshould not be illuminated for the persistent image. As row/columncounters 630 cause frame counter 650 to increment (or decrement) framevalue F by one for each video frame, the match detection is repeatedeach video frame and produces a different result in each video frame foreach pixel (X, Y) of the display device, gradually causing all of thepersistence bits in one-bit-per-pixel persistence display memory 620 tohave the value (e.g., the second value or “0”) that indicates that thecorresponding pixel in the display device should not be illuminated forthe persistent image, thereby erasing the persistent image.

In various embodiments, device 600 may be part of a digitaloscilloscope, a logic analyzer, a digital spectrum analyzer, or othermeasurement instrument which displays data or measurement results on adisplay device. In a beneficial feature, when an instrument includes aplurality of channels (e.g., two channels or four channels) for whichdata is to be measured and a variably persistent image is to bedisplayed, many of the components of device 600 may be shared among themultiple channels, including for example row/column counters 630,pseudorandom pixel value generator 640, frame counter 650, matchdetector 660, and image detector 670. Modifying device 600 to supportmultiple channels requires the addition of additional one-bit-per-pixelpersistence display memories 620 for each additional channel.

In the examples described above with respect to FIGS. 3, 4 and 6 inparticular, embodiments were described where the frame value is strictlyincremented (or decremented) with every frame, e.g. a sequence of (0, 1,2, 3 . . . V−2, V−1, 0, 1, 2, 3, 4, . . . ). This guarantees that thelifetime of each pixel for the persistent image is no greater than Vframes. However in an alternative embodiment, a pseudorandom frame valuefrom {0: V−1} maybe selected for each video frame, e.g. a sequence of(7, V−1, 5, 1, 3, V−3, 0, 3, . . . ). In that case, the lifetime of eachpixel of the display device for the persistent image is variable andpotentially much longer than in the “fixed” case.

Also, in the examples described above with respect to FIGS. 3, 4 and 6in particular, embodiments were described where the pseudorandom pixelvalues were fixed for every frame for a given persistence value Vbecause the pseudorandom pixel value generator was reloaded with thesame seed value for every frame. However in an alternative embodiment,the pseudorandom pixel value generator may be reloaded with a differentseed value each time that the frame counter is reset to zero (i.e., foreach set of V video frames). In that case, all of the pixels of thepersistent image would fade away with one spatial pattern during one setof V video frames, and would fade away with a different spatial patternduring another set of V video frames.

While example embodiments are disclosed herein, one of ordinary skill inthe art appreciates that many variations that are in accordance with thepresent teachings are possible and remain within the scope of theappended claims. The invention therefore is not to be restricted exceptwithin the scope of the appended claims.

The invention claimed is:
 1. A device, comprising: a memory configuredto store a persistence bit for each of a plurality of pixels of adisplay device, the persistence bit having a first value when acorresponding pixel should be illuminated for displaying a persistentimage, and having a second value when the corresponding pixel should notbe illuminated for the persistent image; a pseudorandom pixel valuegenerator configured to receive a seed value for each video frame of thedisplay device and in response thereto to generate pseudorandom pixelvalues for the plurality of pixels during each video frame, eachpseudorandom pixel value being not greater than a specified variablepersistence value corresponding to a desired persistence time for thepersistent image, wherein each pixel of the display device, which has apseudorandom pixel value that matches or is equal to a frame value, isturned off for the persistent image that is being displayed; a framevalue generator configured to output a frame value for each video frameof the display device, each frame value being not greater than thespecified variable persistence value; and a match detector configured tocompare the frame value to the pseudorandom pixel values for theplurality of pixels during each video frame, and for each pixel wherethe comparison indicates a match, to make the persistence bit for thecorresponding pixel have the second value.
 2. The device of claim 1,wherein the specified variable persistence value is a number of videoframes equal to the desired persistence of the persistent image.
 3. Thedevice of claim 1, wherein the pseudorandom pixel value generatorcomprises: a pseudorandom number generator configured to receive theseed value and in response thereto to output a pseudorandom value; and amodulus unit configured to receive the pseudorandom value and thespecified variable persistence value and in response thereto to outputthe pseudorandom pixel values each being an integer not greater than thespecified variable persistence value.
 4. The device of claim 1, whereinthe frame value generator comprises a counter configured to be loadedwith the specified variable persistence value and to count up or downonce for each video frame.
 5. The device of claim 4, wherein thepseudorandom pixel value generator is configured to receive a differentseed value each time the counter resets.
 6. The device of claim 1,wherein the pseudorandom pixel value generator is configured to receivethe same seed value for each video frame.
 7. The device of claim 1,further comprising a second memory storing a second persistence bit foreach of the plurality of pixels, the second persistence bit having thefirst value when a corresponding pixel should be illuminated fordisplaying a second persistent image, and having a second value when thecorresponding pixel should not be illuminated for the second persistentimage, wherein the match detector is further configured, for each pixelwhere the comparison indicates a match, to make the second persistencebit for the corresponding pixel in the second memory have the secondvalue.
 8. The device of claim 1, comprising a measurement instrument,including: the display device; at least one input for receiving at leastone signal to be represented on the display device; and a variablepersistence control input configured to receive a user input specifyingthe variable persistence value corresponding to a desired persistencetime for displaying the persistent image.
 9. The device of claim 8,further comprising a second memory for storing data representing thereceived signal and to be displayed on the display device.
 10. Thedevice of claim 1, wherein the memory stores exactly one persistence bitfor each pixel of the display device.
 11. A method, comprising: storingin a memory a persistence bit for each of a plurality of pixels of adisplay device, the persistence bit having a first value when acorresponding pixel should be illuminated for displaying a persistentimage, and having a second value when the corresponding pixel should notbe illuminated for the persistent image; establishing a variablepersistence value representing a number of video frames of persistencefor displaying the persistent image; and during each video frame of thedisplay device: generating a frame value not greater than the variablepersistence value, generating for each of the plurality of pixels apseudorandom pixel value not greater than the variable persistencevalue, and for each pixel whose pseudorandom pixel value matches theframe value, making the persistence bit for the corresponding pixel havethe second value, wherein each pixel of the display device which has thepseudorandom pixel value that matches or is equal to the frame value isturned off for the persistent image that is being displayed.
 12. Themethod of claim 11, wherein the specified variable persistence value isa number of video frames equal to the desired persistence of thepersistent image.
 13. The method of claim 11, wherein generating thepseudorandom pixel value comprises: generating a pseudorandom value inresponse to the seed value; and in response to the pseudorandom valueand the specified variable persistence value, outputting thepseudorandom pixel values each being an integer not greater than thespecified variable persistence value.
 14. The method of claim 11,wherein generating the frame value comprises: loading a counter with thespecified variable persistence value; and counting up or down once foreach video frame.
 15. The method of claim 14, wherein the pseudorandompixel value generator is configured to receive a different seed valueeach time the counter resets.
 16. The method of claim 11, wherein thepseudorandom pixel value generator is configured to receive the sameseed value for each video frame.
 17. The method of claim 11, furthercomprising: storing in a second memory a second persistence bit for eachof the plurality of pixels, the second persistence bit having the firstvalue when a corresponding pixel should be illuminated for a secondpersistent image, and having a second value when the corresponding pixelshould not be illuminated for the second persistent image; and duringeach video frame of the display device, for each pixel whosepseudorandom pixel value matches the frame value, making the secondpersistence bit for the corresponding pixel have the second value. 18.The method of claim 11, further comprising receiving a user inputspecifying the variable persistence value corresponding to the desiredpersistence time for the persistent image.
 19. The method of claim 18,further comprising: receiving at least one signal; storing in a secondmemory data representing the received signal; displaying on the displaydevice a representation of the at least one signal produced by the datain the second memory.
 20. The method of claim 11, wherein storing in amemory a persistence bit for each of a plurality of pixels of a displaydevice comprises storing in the memory exactly one persistence bit foreach pixel of the display device.